Adaptive UI for the headerbar
When the window gets narrower than 700px, most of the buttons in the headerbar move to a new actionbar at the bottom of the window. This allows the window to continue to shrink, down to 300px if the routing sidebar is not open.
Based on this mockup: https://gitlab.gnome.org/Teams/Design/app-mockups/blob/master/maps/maps-mobile.png
Implements #235 (closed)
Edited by Marcus Lundblad